Switching power supplies are used in a variety of electronic products including computers, calculators, cell phones, cameras, Personal Digital Assistants (PDAs), video game controllers, etc. These types of supplies generate regulated voltages that are used to provide power for the integrated circuits contained in the electronic products. FIG. 1 illustrates a conventional switching power supply 10 that includes a switching regulator 12 having outputs coupled to the gates of switching transistors 14 and 16. A drain terminal of switching transistor 14 is coupled for receiving a source of operating potential VDD and a source terminal of switching transistor 14 is connected to a drain terminal of switching transistor 16. The common connection of the source and drain terminals of switching transistors 14 and 16 is connected to one terminal of an inductor 18. The other terminal of inductor 18 is coupled to an output 20 of switching power supply 10. Output 20 is coupled for receiving a source of operating potential VSS through a capacitor 22 and through a resistor divider network 24. Resistor divider network 24 comprises a resistor 26 having a terminal connected to output 20 and a terminal commonly connected to one terminal of a resistor 28 at a node 30. The other terminal of resistor 28 is coupled for receiving source of operating potential VSS. Node 30 is connected to an input of controller 12 and provides a feedback signal for controlling switching transistors 14 and 16.
In operation, when the output voltage drops below a predetermined value, a feedback signal from node 30 causes switching regulator 12 to generate a control signal, which in turn causes switching transistor 14 to turn on and increase the output voltage, VO, at output 20. In this configuration, the control signal is synchronous with the system clock signal. Because the control signal can last from zero seconds to a full clock period, it can begin and end within a single clock period. However, it cannot be restarted until the next clock signal. A drawback with this system is that if a transient electrical signal causes the output voltage to sag after the control signal has ended, the control signal will not compensate for the sag until the next clock signal. One way to compensate for transient electrical signals that appear in the output signal has been to include a hysteretic control circuit within the controller. Although this technique is capable of quickly compensating for transient electrical signals appearing in the output, it does so without using a clock signal. Thus, the switching frequency of the switching regulator is not constant, but varies with temperature, input voltage, and the components themselves. What's more, because the switching frequency is not constant, the hysteretic control circuitry introduces electromagnetic noise which interferes with the performance of the electronic products. This noise is referred to as Electro-Magnetic Interference (EMI).
Hence, a need exists for a switching regulator and a method for regulating its output voltage with a fast response time while operating at a constant switching frequency using a clock signal.